Published European patent application 472946 describes a polyolefin composition characterized, among other things, by a high flexibility properties (flexural modulus lower than 150 MPa), and comprising a crystalline propylene homopolymer or copolymer, and certain amounts of an elastomeric olefin copolymer soluble in xylene at ambient temperature, which elastomeric olefin copolymer contains less than 40% by weight of ethylene and has an intrinsic viscosity from 1.5 to 4 dl/g.
U.S. Pat. No. 5,300,365 teaches that the above composition is suitable for the manufacture of sheets and films that are substantially gel free and possess excellent physical-mechanical properties.
A method is known in the art for sealing sheets or films of plastic materials, such as for example, vinyl polymers, polyamides, polyesters and polyurethanes, with dielectric heat by radio-frequencies. This method, referred to as the radio-frequency sealing method, consists of placing portions of two or more sheets or films of the above mentioned materials between two electrodes to which is applied an electrical potential difference oscillating at a radio frequency (1-200 MHz), while applying a pressure to the portions of the sheets or films where the sealing is to take place. The heat thus generated in the portions of the polymer sheets or films contacted causes the melting of those portions and allows those portions to adhere and seal. However, this method is not effective to seal sheets or films made of olefin polymers to each other, since such polymers generally develop little dielectric heat in electromagnetic fields at radio frequencies.
In fact, polyolefin films or sheets generally are sealed by applying external heat directly to the portions that must be sealed. However, the seal cycles are longer and the seals thus obtained are non-uniform and of poor quality, when compared with the seal cycles of a radio-frequency apparatus and the seals thereby obtained.
Therefore, it would be a great advantage to have a polyolefin composition with physical-mechanical properties (particularly as to flexibility) substantially the same as those of the composition described in the above mentioned published European patent application 472946, and useful to be converted into sheets and films sealable with dielectric heat generated by radio frequencies.
This invention provides such a composition by melt blending a composition of the type described in the above mentioned European patent application 472946 with a minor amount of at least one polymer capable of developing heat by absorption of radio-frequency electromagnetic radiation.